Jesus had told the people what work they needed to do to obtain eternal life. Now they asked Him what work He would do to prove that He was God"s authorized representative as He claimed to be (cf. 1 Corinthians 1:22). They suggested that producing bread from heaven as Moses did might convince them. Their unwillingness to believe the sign that Jesus had given them the previous day shows the hardness of their hearts. No matter what Jesus did the unbelievers always demanded more. [source][source][source]
Probably Jesus" provision of bread for thousands of people the previous day led them to ask for this greater miracle. Some of them had concluded that Jesus might be the Prophet that Moses had predicted ( John 6:14). If He was, He ought to be able to do greater miracles than Moses did. The manna that Moses produced spoiled if left uneaten overnight, but Jesus seemed to be promising bread that would not spoil. [source][source][source]
The source of the people"s loose quotation is probably Psalm 78:24. However there are also similarities to Nehemiah 9:15; Exodus 16:4; Exodus 16:15; and Psalm 105:40. [source][source][source]
"This section of the discourse is to be understood against the background of a Jewish expectation that, when the Messiah came, he would renew the miracle of the manna." [1][source]