Finding authentic sayings of Jesus in the Gospels is a complex task. Jesus lived and conducted his ministry in the late 20s of the first century, likely being crucified around 30 AD. However, the first Gospel, Mark, is thought to have been written around 70 AD, creating a significant 40-year gap between Jesus' life and the writing of the first Gospel.
During those decades, Christianity evolved from its roots in Judea into the broader Hellenistic world, relying heavily on oral traditions for the transmission of Jesus' story. Oral traditions can be unstable and vary over time unless there is a conserving force in their transmission. Likewise, the Gospels also show discrepancies in chronology and details, such as the timing of the Last Supper and whether Jesus healed two blind men on the way out of Jericho (Matthew) or one man on the way into Jericho (Mark and Luke).
