In summary, this condition is that continuity be maintained between the two prayers; that is, the prayers should follow one another without anything intervening between them except a slight interval sufficient for an iqamah—since a second iqamah is necessary—or for a brief ablution, since one may need to perform ablution between the two prayers, and this is excused.
His statement, “is invalidated,” means: the combining of the prayers.
His statement, “by a regular Sunnah prayer,” means: by performing a regular Sunnah prayer.
His statement, “between them,” means: between the first and second prayers. Thus, if someone combines Maghrib and Isha in a combination performed in advance, then, after praying Maghrib, performs the regular Sunnah prayer of Maghrib, the combining is no longer valid, because the two prayers have been separated by another prayer.
Issue: If he separates them by an obligatory prayer—such as when, after praying Maghrib, he remembers that he had prayed Asr without ablution and so prays Asr—then there is no valid combining. For if combining is invalidated by a regular Sunnah prayer that is connected to one of the prayers being combined, then it is even more readily invalidated by a prayer unrelated to them.
The same applies, all the more so, if he performs a voluntary prayer other than a regular Sunnah prayer. For if it is invalidated by a regular Sunnah prayer connected to the prayer being combined, then a prayer unrelated to it and not prescribed in connection with it is even more clearly invalidating.
Shaykh al-Islam Ibn Taymiyyah chose the view that maintaining continuity between the two combined prayers is not a condition. He said: the meaning of combining is the joining of the two prayer-times—that is, joining the time of the second prayer to that of the first, such that the two times become a single time in the case of an excuse—not the joining of the acts themselves. According to Shaykh al-Islam’s view, if a man prays Dhuhr while travelling without intending to combine the prayers, or if he had been resident and then decides to travel before Asr, he may combine them once he has travelled, even if a lengthy interval has elapsed. According to what the author has stated, however, he may not combine them for two reasons: