Trump actually names the source for this statistic:

If you go to Fusion, you will see a story: About 80% of the women coming in, you know who owns Fusion? Univision! Go to Fusion and pick up the stories on rape.

The Fusion article is available online and explains where the number comes from:

A staggering 80 percent of Central American girls and women crossing Mexico en route to the United States are raped along the way, according to directors of migrant shelters interviewed by Fusion.

This does not seem like a reliable approach to getting correct numbers. Fusion actually also names an Amnesty International report which puts the number at 60%, but also names a study - with a small sample size - that puts the number even lower:

Women and girl migrants, especially those without legal status travelling in remote areas or on trains, are at heightened risk of sexual violence at the hands of criminal gangs, people traffickers, other migrants or corrupt officials. Sexual violence, or the threat of sexual violence, is often used as a means of terrorizing women and their relatives. Many criminal gangs appear to use sexual violence as part of the “price” demanded of migrants. According to some experts, the prevalence of rape is such that people smugglers may require women to have a contraceptive injection prior to the journey as a precaution against pregnancy resulting from rape. It is a widely held view – shared by local and international NGOs and health professionals working with migrant women – that as many as six in 10 migrant women and girls are raped. [21] A study in 2006 interviewed 90 migrant women held in Iztapalapa Migrants’ Detention Centre, of whom just over half were from Central America. Twenty-three women reported experiencing some kind of violence, including sexual violence. Of these, 13 stated the person responsible was a state official. [22] Researchers carrying out the study believed the figures may significantly understate the problem because of the reluctance of women to discuss sexual violence, particularly when they are in detention.

At least the first cited report is available online, but not in English.

The implication Trump is trying to make - that most/many undocumented immigrants are criminals and/or rapists is not supported by this number though. The answer to the question "Who’s doing the raping?" is not "80% of illegal immigrants", but "criminal gangs, people traffickers, other migrants or corrupt officials".