Tafsir Al-Jalalayn: As-Saff, Ayah 2

(O ye who believe) in Muhammad (pbuh) and the Qur'an! (why say ye that which ye do not?) why do you speak about that which you do not act upon? This is because they said: �O Messenger of Allah! If we knew the most beloved deed to Allah, we would certainly do it�. But when he informed them about the works which are dear to Him in His saying (O ye who believe! Shall I show you a commerce that will save you from a painful doom?) whose pain extends to the hearts, they waited for as long as Allah willed, but Allah did not explain further. They then said: �if only we knew what it is so that we give for it our wealth, persons and families�. Allah answered by saying (Ye should believe in Allah and His messenger, and should strive for the cause of Allah with your wealth and your lives�), and so Allah explained to them what to do, saying: you should be firm in your belief in Allah and His Messenger and you should also strive to obey Allah using your wealth and own persons. But when they were tried in the Battle of Uhud and ran away from the battlefield, leaving the Prophet (pbuh) behind, Allah reproached them saying: why do you promise that which you cannot fulfil and why talk about that which you cannot perform?