Excerpt from an address by David O. McKay given in the April 1950 General Conference: 1
Qualifications and calling of missionaries
The physically handicapped. Again there is the problem of the physically handicapped; of course you would like them to go, but think how they would interfere with their companions. Elders should go two by two, and so should sisters, and no one of them should be hampered in his or her activity with somebody who is not equal to the responsibilities of missionary work.