I have been wearing cap in every Salat (prayer) and doing Du'a after every Salat from my childhood, although i do not consider that it is obligation, but some days ago I met somebody they told me that it is Bid'at, and every Bid'at is prohibited in Islam. Is it really Bid'at and we should not wear cap in Salat and do Du'a after Salat?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. Covering the head while performing prayers is likable. According to Kuwaiti Judicial Encyclopedia, "The scholars are in agreement that covering the head for men in prayer is likable, whether with a turban or the like. The Prophet (may Allah's peace and blessing be upon him) used to perform prayers wearing a turban. The Hanafi jurists hold the view that it is dislikable for a man to pray with his head uncovered, if he does so out of laziness then this is a neglect of solemness that contradicts humbleness and submission. Sulaiman Al-Ajeely , in "Hashiyat Al-Jumal", says: Wearing a turban during prayers and for good attire is a Sunnah (a practice of the Prophet, peace be upon him), based on the many Ahadith on this issue. The weakness of many of such Ahadith is amended by the abundance of ways in which they are narrated, and to claim that most of these narrations are fabricated is a matter of indifference. As for making Du'a (supplications) after the prayers, it is a recommended and likable matter and not Bid'ah (innovation). This is evidenced by the following Ahadith: 1- Abu Umamah said: "The Prophet (peace be upon him) was once asked about the most answerable Du'a. He answered: The one made during the last part of the night and at the end of obligatory prayers. [Narrated and classified as acceptable by At-Tirmizi ] 2- Mu'ath Ibn Jabal said that the Prophet (peace be upon him) took his hand and said to him: "O Mu'ath! By Allah I love you. By Allah I love you." He then said: "O Mu'ath! I recommend you not to leave saying after each prayer: ' O Allah, help me to remember You, show gratitude to You, and worship You in the best way. Mu'ath recommended this for As-Sanabihi, who in turn recommended it to Abu Abdur-Rahman . [Narrated by Ahmad , An-Nasai'e , Abu Dawood , and others]. This narration was ruled by Al-Hakim as authentic on the condition of the two Sheikhs ( Al-Bukhari and Muslim ). Al-Hafith Ibn Hajar also mentioned a collection of Ahadith other than these aforementioned ones, which may be seen in "Al-Fath". Allah knows best.