Is it permissible for a man to say to a Muslim woman, "I love you for the sake of Allah"?

Summary: The hadith cited by the questioner is authentic. The origin of love between a man and a woman is found between spouses and mahrams (unmarriageable kin), and the Shari'ah blocks avenues to fitnah (temptation/strife) between non-mahrams.

If a man is dedicated to good deeds and a woman's heart becomes attached to him out of love for Allah, there is no harm in this, as long as the matter is confined to loving good and its people. The reward for those who love each other for Allah's sake encompasses everyone, and it is not a condition to inform the other party of this love.

If fitnah is assuredly absent when a woman informs a man of this love (for example, if the shaykh is old and there is no direct communication), then there is no harm in it. Sheikh Ibn Baz, may Allah have mercy on him, permitted this for a woman who asked him via a letter.

One must be cautious about being lenient in saying and hearing this phrase between non-mahrams, as Satan might exploit it to incite fitnah. Scholars have prohibited a young woman from saying "may Allah have mercy on you" when a man sneezes, greeting him, or offering him condolences if he is a non-mahram, for fear of fitnah. The phrase "I love you for Allah's sake" is even more deserving of prohibition if fitnah is anticipated.

This phrase is said by a man to another man, to his wife, and to his mahrams. And by a woman to another woman, to her husband, and to her mahrams. As for between a non-mahram man and woman, the least of its rulings is dislike, unless fitnah and harm are assuredly absent.

What happened with Sheikh Ibn Baz, may Allah have mercy on him, was through a written letter, and this cannot be compared to direct communication (like phone calls) which might open the door to fitnah. Prohibition is necessary in such cases to block the means (to sin) and ward off evil.