Sayyiduna Abdullah bin Ukaym (may Allah be pleased with him) said that when it was the night of doubt, Sayyiduna Umar (may Allah be pleased with him) would stand after the Maghrib prayer and say: This is the month whose fasting Allah has made obligatory upon you, and He has not made its night prayer obligatory. Whoever among you is able to stand in prayer, let him stand, for it is a voluntary good deed that Allah has commanded; and whoever is not able, let him sleep in his bed. Let no one say: If so-and-so fasts, I will fast, and if he stands in prayer, I will stand in prayer. So whoever fasts or stands in prayer, let him do it for Allah. Lessen idle talk in the houses of Allah, and know that as long as one is waiting for the prayer, he is in prayer. Beware! Let none of you precede the month. "Fast upon its appearing and break the fast upon its appearing. If it is clouded over you, then count thirty days of Sha'ban." Then do not break the fast until the night has hidden behind the hillocks.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7954