It is narrated from Sulayman bin Muhammad bin Ka'b that Sayyiduna Umar (may Allah be pleased with him) asked Ka'b (may Allah be pleased with him): What expiation did Ka'b give when he shaved his head? He said: "He slaughtered a cow."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16379