Wrap the condition in which:
df[which(df$number1 < df$number2), ]
How it works:
It returns the row numbers where the condition matches (where the condition is TRUE) and subsets the data frame on those rows accordingly.
Say that:
which(df$number1 < df$number2)
returns row numbers 1, 2, 3, 4 and 5.
As such, writing:
df[which(df$number1 < df$number2), ]
is the same as writing:
df[c(1, 2, 3, 4, 5), ]
Or an even simpler version is:
df[1:5, ]