How did Allah decree that Muhammad (peace be upon him) should fast during Ramadan?
In the year 2 AH, Allah sent down ayat 183 (يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ كُتِبَ عَلَيْكُمُ ٱلصِّيَامُ كَمَا كُتِبَ عَلَى ٱلَّذِينَ مِن قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ) which means in English (O you who believe! Fasting (aṣ-Ṣiyām) has been prescribed for you as it was prescribed for those before you, so that you may attain piety). Thus, Muhammad (peace and blessings be upon him) understood that fasting was obligatory. Furthermore, verse 184 explains the exceptions (for a specified number of days. Whoever among you is ill or on a journey, then an equal number of other days must be fasted. But for those who are unable to fast (except with great difficulty), there is a compensation: feeding a poor person. And whoever does more of his own volition, it is better for him; but to fast is better for you, if you only knew!). Allah also explained the reason why we fast during the month of Ramadan and not another month (The month of Ramadan is that in which the Quran was revealed as guidance for mankind and clear proofs of guidance and criterion. So whoever of you is present in the month, let him fast; and whoever is ill or on a journey, then an equal number of other days. Allah intends for you ease and does not intend for you hardship, so that you may complete the prescribed period and glorify Allah for having guided you and that you may be grateful). This means that Muhammad (peace and blessings be upon him) only fasted nine Ramadans because he unfortunately died in the 11th year of the Hijra.