Musannaf Ibn Abi Shaybah

Hazrat Abu Dharr narrates that my beloved friend, the Holy Prophet (peace and blessings be upon him), instructed me to perform the prayers at their prescribed times. Then, after you have performed your prayer, if you see that people have already prayed, your prayer is secured; and if they have not prayed, then join them—your prayer will become a voluntary (nafl) prayer.