Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The month can also be of twenty-nine nights. Therefore, do not fast without sighting the moon, nor break the fast without sighting the moon, except if clouds cover you. Then, if it is cloudy, estimate and count the days of the month." Imam Abu Bakr (may Allah have mercy on him) says that Ismail bin Ja'far was one of the greatest hadith memorisers of his time in the world.