Mark 8:16
And they reasoned among themselves, saying, It is because we have no bread.
Cross-reference
Matthew 16:7
Parallel
Matthew 16:7 is the parallel account where the disciples also misunderstand Jesus' warning about leaven, thinking it concerns physical bread.
Matthew 16:8
Parallel
Matthew 16:8 records the same discussion with Jesus' rebuke about lack of bread, directly paralleling this account.
Matthew 14:20
Contrast
Matthew 14:20 records the feeding of 5000, a miracle the disciples witnessed that directly contrasts their worry about lacking bread.