Is it better to pray supererogatory prayers at home or in the mosque, and what is the evidence for that?

It is better to pray supererogatory (nafila) prayers at home, unless they are sunnah to be performed in congregation in the mosque, such as the eclipse prayer (Salat al-Kusuf), or if there is encouragement for performing them in the mosque, like the supererogatory prayers before the Friday prayer. Many pieces of evidence have been reported to support this, including: "Make some of your prayers in your houses and do not turn them into graves," and "The best prayer is a person's prayer in his house, except for the obligatory prayers," and Aisha's saying: "He (the Prophet) used to pray four rak'ahs in my house before Dhuhr, then go out and lead the people in prayer," and "When one of you finishes prayer in his mosque, let him set aside a portion of his prayer for his home, for Allah will place good in his home because of his prayer."

The wisdom behind this is that praying at home is closer to sincerity and further from ostentation (riya'), and it is considered a secret good deed, in addition to reminding and teaching the household members. As for the eclipse prayer and the supererogatory prayers before the Friday prayer, it is recommended to perform them in the mosque.