Is this hadith authentic? And could you please give me the whole hadith that describes the straight path as worshipping Allaah alone, telling the truth even if a near relative is concerned, not killing your child out of poverty - and I think that there was another one; can you please give me this hadith?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

We are not aware of any hadeeth that includes all these words or their meanings in one single hadeeth, but there are various texts about these meanings.

Telling the truth even against relatives is mentioned in a Quranic verse; Allah says (what means): {O you who have believed, be persistently standing firm in justice, witnesses for Allah, even if it be against yourselves or parents and relatives.} [Quran 4:135]

Ibn Katheer said in his Tafseer (exegesis), “{...Even if it be against yourselves} means: say the truth even if you would be harmed as a consequence ... {or parents and relatives} means: even if you have to testify against your parents and kin. Do not compromise for their sake; rather, say the truth even if they would be harmed in the process...”

The prohibition of killing children out of fear of poverty is mentioned in the Quranic verse (which means): {And do not kill your children for fear of poverty.} [Quran 17:31]

Interpreting the Straight Path to mean the worship of Allah alone is correct because Islam is based on the worship of Allah alone, and the Prophet interpreted the Straight Path as being Islam, as in the hadeeth of An-Nawwaas ibn Sam’aan Al-Ansaari, may Allah be pleased with him, in which the Prophet said:

“Allah has set an example: a Siraat (straight path) that is surrounded by two walls on both sides, with several open doors in the walls covered with curtains. There is a caller at the gate of the Siraat who heralds, ‘O people! Stay on the path and do not deviate from it.’ Meanwhile, a caller from above the path is also warning any person who wants to open any of these doors, ‘Woe unto you! Do not open it, for if you open it you will pass through.’ The straight path is Islam, and the two walls are Allah's set limits, while the doors resemble what Allah has prohibited. The caller at the gate of the Siraat is the Book of Allah (the Quran), while the caller above the Siraat is Allah's preacher in the heart of every Muslim.” [Ahmad, and Al-Haakim in his Mustadrak]

Ibn katheer said in his Tafseer:

“With regard to the straight path, Imaam Abu Ja’far ibn Jareer, said, ‘All the interpreters of the Quran of the entire Ummah (Muslim nation) agreed in a consensus that the Straight Path is the clear way that has no crookedness ... then the expressions of the interpreters of the Quran from the Salaf (righteous predecessors) and the Khalaf (the successors, the latecomers) differed about the interpretation of the word ‘Siraat’; but they all mean one thing, which is to follow Allah and His Messenger; so it was narrated that it means the Book of Allah ... and it was said: it is Islam ... Mujaahid said: '{Guide us to the Straight Path} means: to the Truth.' This is more comprehensive, and there is no contradiction between this and what is mentioned before ... Abu al-‘Aaliyah narrated: '{Guide us to the Straight Path} means: to the Prophet and his two Companions after him (Abu Bakr and ‘Umar, may Allah be pleased with them)…' All these statements are correct, and they are correlated, because if one followed the Prophet and followed the example of those who came after him, Abu Bakr and ‘Umar, then he has indeed followed the Truth, and whoever followed the Truth, he has indeed followed Islam, and whoever followed Islam, he has indeed followed the Quran, which is the Book of Allah and His Strong Rope, and His Straight Path. So all the above expressions are correct and confirm each other; all Praise be to Allah…’” [Abridged]

Allah knows best.