It was Moses, to whom the commandments you observe were revealed by God, who acknowledged Him as the God of Abraham, Isaac, and Jacob.
Regarding the remark “Some of the scribes,” it is likely these scribes were not members of the Sadducees, who would be unlikely to publicly commend the Lord for His insightful response. These individuals were possibly Pharisees, who, despite once collaborating with the Sadducees in their attempts to challenge the Lord, were likely pleased to see their rivals bested (cf. Matt. 22:34 ff.). The Gospel writer Mark notes that a scribe, upon questioning the Lord, recognized the wisdom in His rebuttal of the Sadducees (Mk. 12:23) and subsequently voiced his approval (Mk. 12:32). This particular scribe, along with others sharing his perspective, was among those who, after the Lord's reply to the Sadducees, said to Him, “Master, you have spoken well.” The Lord expertly demonstrated the truth of the resurrection through Scripture, prompting even His adversaries to honor Him at that moment (Lk. 20:40-44).
The Lord's inquiry regarding the Messiah and His lineage is similarly presented across the Gospels of Matthew and Mark. See Matthew 22:41-46 and Mark 12:35-37, where it is noted that “They dared not ask Him.” According to Matthew and Mark, following the embarrassment of the Sadducees, a scribe sought clarification about the greatest commandment in the law (Matthew 22:35-40, Mark 12:28-34). After this exchange, both Mark and Matthew note that no one ventured to pose further questions, particularly following the Lord’s interrogation about whose Son the Messiah is. As Luke excludes the discussion between the Lord and the scribe concerning the greatest commandment—indicating that the question posed by the Lord was self-originated and not prompted by others—he notes that they refrained from further inquiry after His response to the Sadducees regarding the resurrection. In response to “How then is He Son unto him?” Luke refrains from detailing the audience's reaction, a narrative elaborated upon by Matthew and Mark, with Matthew mentioning that no one could respond, while Mark recounts that the crowd listened with joy.