It is narrated from Sayyiduna Jabir (may Allah be pleased with him): the Prophet (peace and blessings be upon him) said: "Completing a good deed is more excellent than beginning it."
Al-Tabarani recorded it in al-Mu'jam al-Saghir; its chain contains a narrator, Abd al-Rahman bin Qays al-Dabbi, who is matruk (abandoned).