Sayyiduna Yazid bin ‘Amir (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said to me:“When you come (to the mosque) for prayer and find the people performing prayer, then perform prayer with them. If you have already performed that prayer, then it will be counted as a voluntary (nafl) prayer for you, and it will be considered as obligatory (fard).”