مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1930
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn "Abbas: I am surprised at those who anticipate the month when the Messenger of Allah (ﷺ) said: “Do not fast until you see it" or he said “Fast when you see it.”
Musnad Ahmad 1931Sahih because of corroborating evidence]