Fasting is mentioned ...

In the Old Testament:

" ... and they fasted that day, ..." 1 Sam 7:6
" ... and they mourned and wept and fasted until evening." 2 Sam 1:12
" ... and proclaimed a fast throughout all Judah." 2 Chron 20:3
" ... Then I proclaimed a fast there at the river of Ahava." Ezra 8:21
" ... I was fasting and praying before the God of heaven." Nehem 1:4
" ... the children of Israel were assembled with fasting." Nehem 9:1

My knees are weak through fasting (Psalm 109:24)

" ... So the people of Nineveh believed God, and proclaimed a fast." Jonah 3:5
" ... Blow the trumpet in Zion Consecrate a fast Call a sacred assembly." Joel 2:15
" ... When you fasted and mourned in the fifth and seventh months ..." Zech 7:5
" ... Thus says the Lord of hosts: The fast of the fourth month, The fast of the fifth, The fast of the seventh, And the fast of the tenth ..." Zech 8:19

In the New Testament

" ... And when He had fasted forty days and forty nights, afterwards He was hungry." Matt 4:2
" ... Moreover, when you fast, do not be like the hypocrites, ..." Matt 6:16
" ... But the days will come when the bridegroom will be taken away from them, and then they will fast." Matt 9:14-15
" ... Then, having fasted and prayed..." Acts 13:2-3
" ... and prayed with fasting, they commanded them to the Lord in whom they had believed." Acts 14:23
" ... that you may give yourselves to fasting and prayer..." 1 Cor 7:5