The conditions for combining by advancing are three:

1. He must begin with the earlier prayer. If he performs both prayers and then discovers that the earlier one was invalid, the second prayer becomes invalid as well.

2. He must intend to combine. This intention should be formed at the beginning of the first prayer, although, according to the more apparent view, it may still be made during that prayer.

3. The two prayers must follow one another without a lengthy break. If the interval becomes long—even for a legitimate excuse—he must postpone the second prayer until its own time. A brief pause does no harm; what counts as “lengthy” is determined by common usage.

A worshipper performing tayammum (dry ablution) may combine, according to the sound opinion, and a brief search for water in between does not harm the combination. If he later learns that he omitted a pillar of the first prayer, both prayers are void and must be repeated in combination. If the omission was in the second prayer, and no lengthy interval has passed, he may correct it; otherwise, that prayer is invalid and no combination is allowed. Even if he was unaware of the omission, he must redo both prayers in their respective times.

When the first prayer is deliberately delayed so that the combination is by postponement, order, continuity, and even the prior intention to combine are not required, according to the sounder view. The delay, however, must have been undertaken with the intention of combining; if not, he incurs sin and the prayer is deemed a make-up.

If he combines by advancing and becomes a resident between the two prayers, the combination is null. Should residency occur during the second prayer or after it, the combination is not nullified, according to the more correct opinion. If he combines by delaying and settles after completing the two prayers, that has no effect; but if he settles before they are finished, the first prayer counts only as a make-up.

Combining on account of rain is permissible by advancing, while the later opinion forbids combining by delaying for rain. For advanced combination, rain must be present at the start of the first prayer, and the sounder view stipulates that it continue until the salām of that prayer. Melted snow and hail take the same ruling as rain. The stronger opinion restricts this concession to someone praying in congregation at a distant mosque who would be harmed by the rain on the way.