It is narrated from Hazrat Zaid bin Khalid (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: The best testimony is that one is ready to give testimony (for the truth) before being asked to do so.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21673
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد منقطع، عبدالله بن عمرو لم يسمعه من زيد بن خالد