Ex 4:18 So Moses went and returned to Jethro his father-in-law, and said to him, "Please let me go and return to my brethren who are in Egypt, and see whether they are still alive." And Jethro said to Moses, "Go in peace."

Jethro was Moses' employer and father-in-law. Out of respect for him, he informed him about his plans. Moses had a moment of unbelief by saying that he would go back to Egypt to see whether his brethren, the Israelites, were still alive. Of course, they were still alive! God told him so. Apparently, he suffered a degree of unbelief here.