Mishkat al-Masabih 18 The Offices of Commander and Qadi كتاب الإمارة والقضاء 102

وَعَنْ زَيْدِ بْنِ خَالِدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَلَا أُخْبِرُكُمْ بِخَيْرِ الشُّهَدَاءِ؟ الَّذِي يَأْتِي بشهادتِه قبلَ أنْ يسْأَلهَا» . رَوَاهُ مُسلم

Zaid b. Khalid reported God’s Messenger as saying, “Shall I not tell you of the best witness? He is the one who produces his deposition before he is asked for it.” Muslim transmitted it.