He saw the disciples straining at the oars, because the wind was against them.  About the fourth watch of the night he went out to them, walking on the lake.  He was about to pass by them, but when they saw him walking on the lake, they thought he was a ghost.  They cried out, because they all saw him and were terrified.  Immediately he spoke to them and said, “Take courage!  It is I.  Don’t be afraid.”

Mark 6:48-50 New International Version