Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "May Allah have mercy on the person who performs four (voluntary) rak‘ahs before ‘Asr."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب صلاة التطوع قبل الصلوات المكتوبات وبعدهن / 1193