It is narrated from Sayyiduna Zaid bin Khalid (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Shall I not tell you about the best of witnesses? He is the one who is ready to give testimony (for the truth) before he is asked to do so."
Hadith Referenceمسند احمد / مسند الشاميين / 17040
Hadith Gradingحکم دارالسلام:حديث صحيح، م: 1719، وهذا إسناد اختلف فيه على مالك