Assalamu alaikum.The Hadith of fasting is a command"fast when you sight it".Is the Hadith to know prayer timings by using stick and see shadow of sun also command?or is it just an advice?I want to know this as some people are saying since we calculate prayer timings well in advance and we are not following the Sunnah of Prophet(peace and blessings of Allah be upon him)by using sticks,we can also calculate month in advance by looking at the Lunar Chart,as Allah says not only sun but moon is also for calculating..I would be happy if you could give the Arabic text with Book,chapter and number.Was Salaam.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The beginning of the month of Ramadan is confirmed by sighting its moon. If on the night of the 29th of Sha’baan the moon is not sighted, the scholars differed in opinion whether or not the opinion of the people who rely on calculations and astronomic estimations should be taken into account. The most preponderant opinion in our view is that these calculations should not be taken into account because the Prophet said: "…If it is hidden from you (due to clouds), then complete Sha'baan thirty days." [Al-Bukhari]

Similarly, what is taken into account in regard to the beginning of the prayer are the signs which the Islamic legislation has set, like the (calculation according to the) shade in Thuhr and 'Asr prayers, and the setting of the sun in regard to Maghrib prayer, the disappearance of the red twilight in 'Ishaa’  prayer, and the spread of the white light in the horizon in regard to Fajr prayer, and this is the task of the Mu'athin (the person making the call to the prayer). Nonetheless, it might be difficult for him to know these signs in the city because it is lit up in the night, and the question in this case is, would it be permissible for him to rely on the calendars which are designed according to these juristic signs?

It appears that he is allowed to take these calendars into consideration, contrary to the case of fasting; Ibn Qaasim from the Shaafi'ee School of jurisprudence said: 'The signs of the beginning of the prayer are more numerous and much easier than that of the beginning of the month of Ramadan.'

In addition to this, it is a repeated difficulty to find out the time of the prayer based on their signs, but it is not difficult to find the time of fasting.