It is reported that on the first night of Ramadân, ‘Umar – Allâh be pleased with him – would pray Maghrib, then say (to the people):
Sit down. Then he would give a small address: Verily the fasting of this month has been made a duty upon you, and standing in night prayer has not been made a duty upon you, but those amongst you who can stand in prayer should do so, for it is from the extra good deeds about which Allâh told us: so whoever cannot stand in prayer, let him sleep on his bed.
And beware of saying: I will fast if so and so fasts and I will stand in night prayer if so and so stands in prayer. Whoever fasts or stands in night prayer, he must make this for Allâh. And you should know that you are in prayer as long as you are waiting for a prayer.
Minimize any vain or false speech in the houses of Allâh (mosques; he said this two or three times). Let none of you fast a few days before the month (in order to avoid missing the beginning of the month; he said this three times). And do not fast until you see [the crescent of the new month] unless it is overcast. If it is overcast, count [the previous month] as 30 days. Then do not break your fasts until you see the night upon the mountain (i.e. you are sure the sun has set).
‘Abd Al-Razzâq Al-San’ânî, Al-Musannaf article 7748.