Kitáb-i-Aqdas

should any one of His servants intend to travel, he must fix for his wife a time when he will return home   ¶67

If the husband leaves without informing his wife of the date of his return, and no news of him reaches her and all trace of him is lost, Bahá'u'lláh has stated that, should the husband have been aware of the law prescribed in the Kitáb-i-Aqdas, the wife may remarry after waiting a full year. If, however, the husband was unaware of the law, the wife must wait until news of her husband reaches her (Q and A 4).