It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would command the freeing of slaves on the occasion of the eclipse prayer."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4886
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[د ك] عن أسماء. صحيح أبي داود ١٠٧٨: خ.