Hello, @JohnLemphung! This is an interesting question! There are a few possibilities. Keep in mind that all of these are speculative.
The first possibility is that the disciples heard this part of Jesus’ prayer before they fell asleep. Each gospel writer had in mind particular points and messages they wanted to communicate to their readers. In any part of their gospels, they only recorded that which would have helped communicate their message. That is why each gospel is different. Jesus may have been in the garden praying for quite a long time, and the gospel writers only recorded the part of the conversation and events they thought would be important for their readers.
The second possibility is that, just as the creation events were communicated supernaturally to the author of Genesis, the prayer of Jesus may have been communicated through the Holy Spirit. It could have also come up in an unrecorded part of a conversation the risen Jesus had with his disciples.
The other possibility is that there may have been someone else around in the garden. The Scripture doesn’t explicitly say there was no one else around.
The truth is that we don’t know. What we do know, however, is that all of these possibilities are valid possibilities.That means the fact that the disciples slept during at least part of Jesus’ prayer time does not undermine the plausibility of the recording of Jesus’ words.
Thanks for the great question…let me know your thoughts!